.asoundrc:

pcm.default pcm.dmixer
pcm.!default {
        type plug
        slave.pcm "dmixer"
}
pcm.dmixer {
     type dmix
     ipc_key 1024
     slave {
        pcm "hw:0,0"
        period_time 0
        period_size 1024
        buffer_size 8192
        rate 48000 # set to your source rate
        periods 128 # dito.
        format S32_LE
     }
        bindings {
            0 0
            1 1
        }
}
ctl.dmixer {
    type hw
    card 0
}

pcm.dsp {
    type plug
    slave.pcm "dmixer" # use our new PCM here
}
pcm.dsp0 {
    type plug
    slave.pcm "dmixer" # use our new PCM here
}
# mixer0 like above
ctl.mixer {
    type hw
    card 0
}

ctl.mixer0 {
    type hw
    card 0
}